1. Field of the Invention
The present invention relates to the charge control of a secondary battery, and especially relates to a device by which a value of the voltage of a secondary battery is detected and the discharge is stopped when the value has reached a predetermined voltage.
2. Description of the Related Art
An electric motor car (including a hybrid powered automobile) obtaining the total or a part of the driving force of the vehicle by an electric motor, has a secondary battery (hereafter, referred to simply as a battery) mounted, and by the electric power accumulated in this battery, said electric motor is driven. Regenerative braking is a characteristic function in such electric motor cars. In regenerative braking, during vehicle braking, the kinetic energy of the vehicle is transformed into electric energy by making said electric motor function as an electricity generator. The obtained electric energy is accumulated in the battery and is reused for acceleration or other vehicle needs. Accordingly, with regenerative braking, it is possible to reuse energy which is normally radiated as thermal energy in an automobile running only by a conventional internal combustion engine, and the efficiency of the energy can considerably be improved.
Here, in order to effectively store the electric power generated during regenerative braking in a battery, it is necessary for the battery to have a corresponding capacity. Furthermore, in a hybrid powered automobile of a type in which it is possible that the generator is driven by the heat engine mounted on the automobile to generate the electric power and this electric power is accumulated in a battery, the electric power accumulated in the battery, that is, the charged amount can freely be controlled. Consequently, in such a hybrid powered automobile, it is desirable that the charged amount of a battery be controlled so that the charged amount may be approximately in the middle state (50.about.60%) between the state of full charge (100%) and the state of no charge (0%), so as to make it possible to receive the regenerative power, and so as to make it possible to supply the electric power to the electric motor immediately if a request is made.
Depending on the running state of a hybrid powered automobile, a situation wherein a great deal of electricity is discharged and the charged amount grows very small may arise. Often, in today's vehicles, there are cases where the battery degrades until the charged amount becomes 0. It is therefore necessary to stop the discharge before that.
Therefore, for example, in Japanese Patent Laid-Open Publication No. Hei 8-185892, a device is described by which the discharge is stopped when a secondary battery is discharged and the voltage of the battery is lowered to a specified value. Accordingly, by this device, the over discharge of a secondary battery is prevented so that the degradation of the battery may be prevented.
However, in the above device, a specified value to stop the discharge of a battery is set to a value of a proper ratio relative to the rated voltage of the battery. Therefore, in this value, a considerable margin is included, and accordingly, this results in that the discharge of a battery being stopped when discharge could still actually be performed. Consequently, it has been impossible that the ability of a secondary battery be effectively utilized to the utmost.